Oil pressure problem 2006 4.8 Vortec

I just purchased a 2006 Express 2500 Van it has 220,000 miles but looks VERY well kept and was a one owner vehicle. I normally don't buy 3/4 ton vans because the contractors that generally use them beat them to death then sell them.

The oil pressure fluctuates and is low once the engine is warm and the lifters will start to chatter but not all the time... this is what has me stumped. Normally you either have good pressure or you don't... it doesn't come and go at least in my experience.
On a google search there was a link to a forum and one of the members said he had "stuff" (sludge, etc.) floating around in the pan and it would block or restrict the pickup at times sounded a but far fetched to me but possible. I was thinking along the lines of a stuck bypass valve or broken bypass valve spring.
Or is it possible that the pump is worn and just needs replacing?

Thoughts?

I have attached a few photos of the truck... as you can see it is in excellent condition and I would assume the engine, trans. were serviced the same.

I would look at dropping the pan and replacing the pump, clean out as much of the sludge as possible at the same time.
Very clean looking for the mileage, if the engine proves to be extremely worn the rest of the truck is definately a candidate for a replacement engine.

Went to my buddies shop and the opinion of the collective motorheads including the guy that owns the machine shop we use was first change the oil, put a good filter on it and a mechanical gauge to verify the pressure.
I did so using an AC/Delco filter and 5-20 Valvoline High Mileage. He didn't have the proper adapter for the gauge so I just replaced the sender.

No Change and probably worse because someone had put heavier weight oil or an additive in before.

Dropped the pan... Pickup is and the entire inside of the engine is as clean as a whistle.

Pulled the front cover and oil pump... nothing really visually worn in the pump and the housings still have manufacturing machine marks. The only thing noteworthy is the bypass spring is very weak. Never having one of these pumps apart we are not sure if that is normal or not. When I took the cap/plug out of the bypass bore I expected to need to keep pressure on the cap to prevent the cap from flying across the shop as we are used to on "regular" oil pumps.
Can't get a pump for a few days at the places I checked (Auto Zone) and NAPA says it is a dealer item? As are the pan and front cover gaskets (Auto Zone has the pan and lists the Timing cover) BTW AZ can get a High Volume/Pressure Melling pump which I would prefer.

I will check a few other local parts houses tomorrow as it was late today (just now) when I got done.

On an up note the 4.8 (and I assume all vortec) engine is a dream to work on! I had the pan out in less than an hour.